/*
 * Copyright (C) 2011 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.ide.vscode.commons;

import java.io.IOException;
import java.lang.reflect.AccessFlag;
import java.util.LinkedHashMap;
import java.util.Map;

import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.TypeAdapter;
import com.google.gson.TypeAdapterFactory;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

/**
 * Adapts values whose runtime type may differ from their declaration type. This
 * is necessary when a field's type is not the same type that GSON should create
 * when deserializing that field. For example, consider these types:
 *
 * <pre>{@code
 * abstract class Shape {
 * 	int x;
 * 	int y;
 * }
 * 
 * class Circle extends Shape {
 * 	int radius;
 * }
 * 
 * class Rectangle extends Shape {
 * 	int width;
 * 	int height;
 * }
 * 
 * class Diamond extends Shape {
 * 	int width;
 * 	int height;
 * }
 * 
 * class Drawing {
 * 	Shape bottomShape;
 * 	Shape topShape;
 * }
 * }</pre>
 *
 * <p>
 * Without additional type information, the serialized JSON is ambiguous. Is the
 * bottom shape in this drawing a rectangle or a diamond?
 *
 * <pre>{@code
 * {
 *   "bottomShape": {
 *     "width": 10,
 *     "height": 5,
 *     "x": 0,
 *     "y": 0
 *   },
 *   "topShape": {
 *     "radius": 2,
 *     "x": 4,
 *     "y": 1
 *   }
 * }
 * }</pre>
 *
 * This class addresses this problem by adding type information to the
 * serialized JSON and honoring that type information when the JSON is
 * deserialized:
 *
 * <pre>{@code
 * {
 *   "bottomShape": {
 *     "type": "Diamond",
 *     "width": 10,
 *     "height": 5,
 *     "x": 0,
 *     "y": 0
 *   },
 *   "topShape": {
 *     "type": "Circle",
 *     "radius": 2,
 *     "x": 4,
 *     "y": 1
 *   }
 * }
 * }</pre>
 *
 * Both the type field name ({@code "type"}) and the type labels
 * ({@code "Rectangle"}) are configurable.
 *
 * <h2>Registering Types</h2>
 *
 * Create a {@code RuntimeTypeAdapterFactory} by passing the base type and type
 * field name to the {@link #of} factory method. If you don't supply an explicit
 * type field name, {@code "type"} will be used.
 *
 * <pre>{@code
 * RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = RuntimeTypeAdapterFactory.of(Shape.class, "type");
 * }</pre>
 *
 * Next register all of your subtypes. Every subtype must be explicitly
 * registered. This protects your application from injection attacks. If you
 * don't supply an explicit type label, the type's simple name will be used.
 *
 * <pre>{@code
 * shapeAdapterFactory.registerSubtype(Rectangle.class, "Rectangle");
 * shapeAdapterFactory.registerSubtype(Circle.class, "Circle");
 * shapeAdapterFactory.registerSubtype(Diamond.class, "Diamond");
 * }</pre>
 *
 * Finally, register the type adapter factory in your application's GSON
 * builder:
 *
 * <pre>{@code
 * Gson gson = new GsonBuilder().registerTypeAdapterFactory(shapeAdapterFactory).create();
 * }</pre>
 *
 * Like {@code GsonBuilder}, this API supports chaining:
 *
 * <pre>{@code
 * RuntimeTypeAdapterFactory<Shape> shapeAdapterFactory = RuntimeTypeAdapterFactory.of(Shape.class)
 * 		.registerSubtype(Rectangle.class).registerSubtype(Circle.class).registerSubtype(Diamond.class);
 * }</pre>
 *
 * <h2>Serialization and deserialization</h2>
 *
 * In order to serialize and deserialize a polymorphic object, you must specify
 * the base type explicitly.
 *
 * <pre>{@code
 * Diamond diamond = new Diamond();
 * String json = gson.toJson(diamond, Shape.class);
 * }</pre>
 *
 * And then:
 *
 * <pre>{@code
 * Shape shape = gson.fromJson(json, Shape.class);
 * }</pre>
 */
public final class RuntimeTypeAdapterFactory<T> implements TypeAdapterFactory {
	private final Class<?> baseType;
	private final String typeFieldName;
	private final Map<String, Class<?>> labelToSubtype = new LinkedHashMap<>();
	private final Map<Class<?>, String> subtypeToLabel = new LinkedHashMap<>();
	private final boolean maintainType;
	private boolean recognizeSubtypes;

	private RuntimeTypeAdapterFactory(Class<?> baseType, String typeFieldName, boolean maintainType) {
		if (typeFieldName == null || baseType == null) {
			throw new NullPointerException();
		}
		this.baseType = baseType;
		this.typeFieldName = typeFieldName;
		this.maintainType = maintainType;
	}

	/**
	 * Creates a new runtime type adapter for {@code baseType} using
	 * {@code typeFieldName} as the type field name. Type field names are case
	 * sensitive.
	 *
	 * @param maintainType true if the type field should be included in deserialized
	 *                     objects
	 */
	public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName, boolean maintainType) {
		return new RuntimeTypeAdapterFactory<>(baseType, typeFieldName, maintainType);
	}

	/**
	 * Creates a new runtime type adapter for {@code baseType} using
	 * {@code typeFieldName} as the type field name. Type field names are case
	 * sensitive.
	 */
	public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType, String typeFieldName) {
		return new RuntimeTypeAdapterFactory<>(baseType, typeFieldName, false);
	}

	/**
	 * Creates a new runtime type adapter for {@code baseType} using {@code "type"}
	 * as the type field name.
	 */
	public static <T> RuntimeTypeAdapterFactory<T> of(Class<T> baseType) {
		return new RuntimeTypeAdapterFactory<>(baseType, "type", false);
	}

	/**
	 * Ensures that this factory will handle not just the given {@code baseType},
	 * but any subtype of that type.
	 */
	@CanIgnoreReturnValue
	public RuntimeTypeAdapterFactory<T> recognizeSubtypes() {
		this.recognizeSubtypes = true;
		return this;
	}
	
	/**
	 * Registers {@code type} identified by {@code label}. Labels are case
	 * sensitive.
	 *
	 * @throws IllegalArgumentException if either {@code type} or {@code label} have
	 *                                  already been registered on this type
	 *                                  adapter.
	 */
	@CanIgnoreReturnValue
	public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type, String label) {
		if (type == null || label == null) {
			throw new NullPointerException();
		}
		if (subtypeToLabel.containsKey(type) || labelToSubtype.containsKey(label)) {
			throw new IllegalArgumentException("types and labels must be unique");
		}
		labelToSubtype.put(label, type);
		subtypeToLabel.put(type, label);
		return this;
	}

	/**
	 * Registers {@code type} identified by its {@link Class#getSimpleName simple
	 * name}. Labels are case sensitive.
	 *
	 * @throws IllegalArgumentException if either {@code type} or its simple name
	 *                                  have already been registered on this type
	 *                                  adapter.
	 */
	@CanIgnoreReturnValue
	public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
		return registerSubtype(type, type.getSimpleName());
	}

	@Override
	public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
		if (type == null) {
			return null;
		}
		Class<?> rawType = type.getRawType();
		boolean handle = recognizeSubtypes ? baseType.isAssignableFrom(rawType) : baseType.equals(rawType);
		if (!handle) {
			return null;
		}

		TypeAdapter<JsonElement> jsonElementAdapter = gson.getAdapter(JsonElement.class);
		Map<String, TypeAdapter<?>> labelToDelegate = new LinkedHashMap<>();
		Map<Class<?>, TypeAdapter<?>> subtypeToDelegate = new LinkedHashMap<>();
		for (Map.Entry<String, Class<?>> entry : labelToSubtype.entrySet()) {
			TypeAdapter<?> delegate = gson.getDelegateAdapter(this, TypeToken.get(entry.getValue()));
			labelToDelegate.put(entry.getKey(), delegate);
			subtypeToDelegate.put(entry.getValue(), delegate);
		}
		
		final RuntimeTypeAdapterFactory<T> thisAdapter = this;
		
		return new TypeAdapter<R>() {

			private TypeAdapter<?> findSubtypeDelegate(Class<?> clazz) {
				if (clazz.isInterface() || clazz.accessFlags().contains(AccessFlag.ABSTRACT)) {
					throw new JsonParseException(
							"cannot serialize/deserialize " + clazz.getName() + "; it is abstract");
				}
				if (baseType.isAssignableFrom(clazz)) {
					TypeAdapter<?> delegate = gson.getDelegateAdapter(thisAdapter, TypeToken.get(clazz));
					labelToDelegate.put(clazz.getName(), delegate);
					subtypeToDelegate.put(clazz, delegate);
					labelToSubtype.put(clazz.getName(), clazz);
					subtypeToLabel.put(clazz, clazz.getName());
					return delegate;
				}
				return null;
			}

			@SuppressWarnings("unchecked") // registration requires that subtype extends T
			@Override
			public R read(JsonReader in) throws IOException {
				JsonElement jsonElement = jsonElementAdapter.read(in);
				JsonElement labelJsonElement;
				if (maintainType) {
					labelJsonElement = jsonElement.getAsJsonObject().get(typeFieldName);
				} else {
					labelJsonElement = jsonElement.getAsJsonObject().remove(typeFieldName);
				}

				if (labelJsonElement == null) {
					throw new JsonParseException("cannot deserialize " + baseType
							+ " because it does not define a field named " + typeFieldName);
				}
				String label = labelJsonElement.getAsString();
				TypeAdapter<R> delegate = (TypeAdapter<R>) labelToDelegate.get(label);
				if (delegate == null && recognizeSubtypes) {
					try {
						delegate = (TypeAdapter<R>) findSubtypeDelegate(Class.forName(label));
					} catch (ClassNotFoundException e) {
						// ignore
					}
				}
				if (delegate == null) {
					throw new JsonParseException("cannot deserialize " + baseType + " subtype named " + label
							+ "; did you forget to register a subtype?");
				}
				return delegate.fromJsonTree(jsonElement);
			}

			@SuppressWarnings("unchecked") // registration requires that subtype extends T
			@Override
			public void write(JsonWriter out, R value) throws IOException {
				Class<?> srcType = value.getClass();
				TypeAdapter<R> delegate = (TypeAdapter<R>) subtypeToDelegate.get(srcType);
				if (delegate == null && recognizeSubtypes) {
					delegate = (TypeAdapter<R>) findSubtypeDelegate(srcType);
				}
				if (delegate == null) {
					throw new JsonParseException(
							"cannot serialize " + srcType.getName() + "; did you forget to register a subtype?");
				}
				JsonObject jsonObject = delegate.toJsonTree(value).getAsJsonObject();

				if (maintainType) {
					jsonElementAdapter.write(out, jsonObject);
					return;
				}

				JsonObject clone = new JsonObject();

				if (jsonObject.has(typeFieldName)) {
					throw new JsonParseException("cannot serialize " + srcType.getName()
							+ " because it already defines a field named " + typeFieldName);
				}
				String label = subtypeToLabel.get(srcType);
				clone.add(typeFieldName, new JsonPrimitive(label));

				for (Map.Entry<String, JsonElement> e : jsonObject.entrySet()) {
					clone.add(e.getKey(), e.getValue());
				}
				jsonElementAdapter.write(out, clone);
			}
		}.nullSafe();
	}
}